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N-Arylamino-1:3-diaza-1:3-butadienes 4 are shown to undergo regioselective reactions with phenyl- and chloroketenes resulting in high yields of 3-aryl-2-methylthio6-phenyl-4(3H)-pyrimidinones 7. Similar reactions with bromo- and iodoketenes, resulted, via aziridinium intermediates 12, in good yields of 3-aryl-S-(N-arylamino)-2-methylthio-6-phenyl4(3H)-pyrimidinones 13, The mechanistic aspects of cycloadditions and semi-empirical AM1 calculations for these diazabutadienes are also reported.

We present a computer program, MORATE (Molecular Orbital RATE calculations), for direct dynamics calculations of unimolecular and bimolecular rate constants of gas-phase chemical reactions involving atoms, diatoms, or polyatomic species. The potential energies, gradients, and higher derivatives of the potential are calculated whenever needed by semiempirical molecular orbital theory without the intermediary of a global or semiglobal fit. The dynamical methods used are conventional or variational transition state theory and multidimensional semiclassical approximations for tunneling and nonclassical reflection. The computer program is conveniently interfaced package consisting of the POLYRATE program, version 4.5.1, for dynamical rate calculations, and the MOPAC program, version 5.03, for semiempirical electronic structure computations. All semiempirical methods available in MOPAC, in particular MINDO/3, MNDO, AM1, and PM3, can be called on to calculate the potential and gradient. Higher derivatives of the potential are obtained by numerical derivatives of the gradient. Variational transition states are found by a one-dimensional search of generalized-transition-state dividing surfaces perpendicular to the minimum-energy path, and tunneling probabilities are evaluated by numerical quadrature.

POLYRATE is a computer program for the calculation of chemical reaction rates of polyatomic species (and also atoms and diatoms as special cases). Version 1.1 was submitted to the CPC Program Library in 1987, and since that time we have considerably improved the program and made it more portable, and we have added several new capabilities, resulting in the present improved version 4. The methods used are variational or conventional transition state theory and multidimensional semiclassical adiabatic and large-curvature approximations for tunneling and nonclassical reflection. Rate constants may be calculated for canonical or microcanonical ensembles or for specific vibrational states of selected modes with translational, rotational, and other vibrational modes treated thermally. Bimolecular and unimolecular reactions and gas-phase, solid-state, and gas-solid interface reactions are all included. Potential energy surfaces may be global analytic functions or implicit functions defined by interpolation from input energies, gradients, and force constants (Hessian matrices) at selected points on a reaction path. The program calculates reaction paths by the Euler, Euler stabilization, or Page-McIver methods. Variational transition states are optimized from among a one-parameter sequence of generalized transition states orthogonal to the reaction path. Tunneling probabilities are calculated by numerical quadrature, using either the centrifugal-dominat-small-curvature approximation, the large-curvature-version-3 approximation, and / or methods that were available earlier. In the large curvature case the tunneling probabilities may be summed over final vibrational states for exoergic reactions or initial vibrational states for endoergic reactions.

We present a computer program for calculating rate constants of gas-phase chemical reactions involving one or two reactants with a total of three to ten atoms. The program accepts information about the potential energy surface in the form of either an analytic potential energy function or a sequence of geometries, energies, gradients and second (or higher) derivative matrices at points along the reaction path. In the former case the program itself calculates the reaction pathe and the sequence of derivative matrices. From this information the program calculates the rate constant for quantized internal degrees of freedom and classical reaction-path motion by variational transition state theory (VTST). The probabilities for tunneling and nonclassical reflection are estimated by semiclassical methods and incorporated by a transmission coefficient, which for thermal reactions is based on the ground state. There are several options for including the effects of anharmonicity in the independent-normal-mode approximation, and the reaction-path curvature may be included in the tunneling calculation by the small-curvature approximation. The article also presents test calculations illustrating the use of new reaction-path interpolation and extrapolation procedures which should be useful in conjunction with VTST calculations based on ab initio gradients and Hessian calculations.

We have applied improved canonical and microcanonical variational transition state theories to the calculation of the rate constants for the polyatomic reaction OH+H2→H2O+H over the temperature range 200–2400 K using the Schatz–Elgersma fit to the Walch–Dunning ab initio potential energy surface. The results are compared to canonical variational transition state theory calculations that employed the same potential energy surface and to experiment. We find that the new results, which are in good agreement with experiment, differ very slightly from those obtained with canonical variational theory. One explanation for this agreement is that the microcanonical variational transition states have a rather weak energy dependence and lie within 0.04a0 of the ground-state adiabatic barrier maximum. We also find that quantum mechanical effects and the inclusion of reaction-path curvature are important at lower temperatures, and that the transition state theory treatment of this reaction breaks down for temperatures higher than about 2400 K.

We have extended the general polyatomic canonical variational theory formalism of Isaacson and one of the authors to improved canonical and microcanonical variational theory. We have calculated the rate constants for the reaction in the title over the temperature range 200–2500 K using all three variational theories and the Melius–Blint ab initio potential energy surface. The results are compared to canonical variational calculations based on the reaction-path interpolation scheme of Quack and Troe, to the trajectory calculations of Miller, and to experiment. We find that the microcanonical variational transition states have a strong energy dependence and the generalized free energy of activation curves have two maxima. Quantization effects appear to be important at the lower temperatures, and recrossing effects may be important at higher temperatures.

Abstract An empirical constraint for the determination of the force constants of a General Valence Force Field for the tetrahedral halides with the general formula AX4 n (where A = an element of the groupe II, III, IV, V and VI; X = F, Cl, Br and I and n = +1, 0, -1 and -2) is presented. The model is defined by a constant value of the parameter p= (ft -ftt) / (ft + 3ftt). The optimum value, p = 0.73, obtained for the mixed halides of chlorobromomethane is compared with those calculated from other force fields commonly used.

Some new relations involving the inertia defect Δ in different vibrational states are given. The utility of such data in fixing a unique force field for XY2(C2v), XY3(D3h), and XY4(D4h) type molecules has been discussed. The parametric study and the extremal properties of Δvib for XY2(C2v) type molecules are also presented. It is shown that in cases where the bonded mean amplitudes do not provide the unique solution for the force constants, the Δ values in different states can be used. The experimentally determined values of Δvib(0) for many XY2(C2v) type molecules are found to be in the neighborhood of their maximum ones, which corresponds to the constraint ζ13y=0 or L21(A1)=0 (upper triangular L matrix).